[Feat] MOQ feedback draft support

[Sy0307]

Summary

Implements the complete delivery feedback mechanism defined in draft-moq-delivery-feedback-00 on top of xquic's existing MoQ draft-14 protocol stack. This covers receiver-side delivery status tracking, Feedback Report encoding/decoding, sender-side CC decision-making, and cross-layer control integration at the QUIC CC layer. All logic is built directly as extensions to the moq_transport real protocol stack, reusing the existing session/track/message framework.

Architecture

Receiver (Subscriber)                        Sender (Publisher)
+-----------------------+                    +----------------------------+
| feedback_tracker      | -- fb_report -->   | feedback_track (on_object) |
| (per-object status    |    (encoded &      |   -> fb_report decode      |
|  + LATE/LOST verdict) |     sent via       |   -> fb_input computation  |
+-----------------------+     feedback track  |   -> decision callback     |
| fb_report_gen         |     unidirectional) |      or auto decision      |
| (timer-driven report  |                    |   -> crosslayer gateway    |
|  generation, 100ms)   |                    |      (clamp/rate-limit)    |
+-----------------------+                    |   -> xqc_conn_signal       |
                                             |   -> BBR override          |
                                             +----------------------------+

Changes by Layer

1. Transport / CC Layer Extensions

File	Changes
include/xquic/xquic.h	Added on_recv_timestamp and x_layer_app_event callbacks to CC interface; added xqc_conn_signal_x_layer_app_event() public API
include/xquic/xquic_typedef.h	Defined cross-layer event types (XQC_APP_EVENT_PACING_GAIN_UPDATED, etc.) and payload structs (xqc_pacing_gain_update_t, etc.); all payloads carry expire_time
include/xqc_types.h	Defined xqc_crosslayer_event_t as the unified event interface for the crosslayer gateway
src/transport/xqc_conn.c	Implemented xqc_conn_signal_x_layer_app_event() to dispatch cross-layer events to the CC algorithm
src/transport/xqc_frame.c	ACK_EXTENDED frame extension support
src/transport/xqc_send_ctl.c/.h	Receive timestamp callback integration
src/congestion_control/xqc_bbr.c/.h	Added MoQ override mechanism to BBR (pacing_gain / pacing_rate / target_bitrate — three override types with automatic expiry recovery); added on_recv_timestamp callback for OWD recording
src/cc/xqc_crosslayer.c/.h	Cross-layer control gateway: unified entry point with rate-limiting (50ms), gain clamping [0.5, 2.0], dispatch counting, and logging

2. QUIC Receive Timestamps

File	Changes
src/recv_ts/xqc_recv_ts.c/.h	Implemented draft-ietf-quic-receive-ts per-packet receive timestamp ring buffer with delta encoding/decoding
include/xquic/xqc_configure.h	Compile-time feature flag

3. MoQ Feedback Report Codec

File	Changes
include/moq/xqc_moq_fb_report.h	Defined xqc_moq_fb_report_t, xqc_moq_fb_object_row_t, xqc_moq_fb_summary_stats_t, xqc_moq_fb_optional_metric_t, strictly aligned with draft Section 5 format
moq/moq_transport/xqc_moq_fb_report.c	Implemented encode/decode/free using ZigZag + QUIC varint encoding for signed deltas
src/common/xqc_varint.c/.h	QUIC varint encoding/decoding utilities

4. Receiver: Delivery Tracking + Report Generation

File	Changes
moq/moq_transport/xqc_moq_feedback_tracker.c/.h	Ring buffer tracker recording per-object status (RECEIVED / RECEIVED_LATE / NOT_RECEIVED). LATE verdict uses gap-based interval detection (avoids timeline drift). Tail loss timeout detection (2× expected_interval). Summary stats computed per-period (current report cycle only)
moq/moq_transport/xqc_moq_fb_report_gen.c/.h	100ms timer-driven: exports object rows + summary from tracker, encodes into fb_report, and sends via feedback track unidirectional stream
moq/moq_transport/xqc_moq_message_handler.c	Calls fb_report_gen_on_media_object_received() to update tracker upon receiving a media Object

5. Sender: Feedback Consumption + CC Decision

File	Changes
moq/moq_transport/xqc_moq_feedback_track.c/.h	Feedback track ops: decode report → compute fb_input (loss_rate, late_rate, playout_ahead, estimated_bw) → trigger observer callback → user decision callback (Level 2.5) → auto decision fallback → crosslayer dispatch
moq/moq_transport/xqc_moq_feedback_decision.c/.h	Pure policy layer: 7 priority rules (severe loss → critical playout → heavy loss/late → mild loss/late → BW hint → recovery probe-up), with configurable thresholds

6. Session / Track Extensions

File	Changes
include/moq/xqc_moq.h	Added public APIs: feedback decision types, 3-level control configuration APIs, crosslayer bounds API, diagnostic getter APIs; added on_delivery_feedback / on_feedback_decision callbacks
moq/moq_transport/xqc_moq_session.c/.h	Session initialization of crosslayer gateway, negotiation of DELIVERY_FEEDBACK parameter (0xA2, bitmap 0x07), feedback state management
moq/moq_transport/xqc_moq_track.c/.h	Track: added feedback_tracker pointer and target_latency_us field
moq/CMakeLists.txt	Added all feedback source files

7. Demo + E2E Tests

File	Changes
moq/demo/xqc_moq_feedback_client.c	Demo client: subscribes to video track + sets target_latency + creates feedback track to send reports
moq/demo/xqc_moq_feedback_server.c	Demo server: publishes video track + subscribes to feedback track + callback logging + commented example code (Level 0/1/2 usage patterns)
scripts/moq_scripts/moq_feedback_test.sh	18 E2E test cases using Linux tc for network impairment injection, all passed

E2E Test Cases (18/18 passed)

#	Case	Draft Section Coverage
1	clean_network	S4 (Setup), S5 (Report)
2	network_loss	S5.3 (NOT_RECEIVED)
3	jitter_tolerance	S5.4 (avg_inter_arrival_delta)
4	negotiation	S4 (0xA2 parameter)
5	network_delay	S5.4 (Summary Stats)
6	heavy_loss (40%)	S5.3, S10.1 (best-effort)
7	feedback_loss	S10.1 (report seq gap)
8	summary_accuracy	S5.4.2 (total == recv + lost)
9	playout_metric	S5.5 (PLAYOUT_AHEAD_MS)
10	loss_recovery	S7.2 (status update)
11	late_via_delay	S5.3 (RECEIVED_LATE)
12	report_seq	S5.1 (monotonic sequence)
13	object_entries	S5.2 (object_id strict ascending)
14	report_freq	S7.3 (50ms - 2s interval)
15	cc_integration	S8.4 (crosslayer → BBR override)
16	override_expiry	BBR override auto-expire
17	cubic_no_handler	cc_dispatch==0 for CC without handler
18	cc_recovery_probeup	last_gain > 1.0 after recovery

Feedback Control API (3 Levels)

Level	Code Required	Description
0 (Default)	Zero code	Built-in auto decision + default thresholds
1 (Custom thresholds)	1 line	set_feedback_decision_config() to tune thresholds
2 (Decision callback)	1 callback	on_feedback_decision for fully custom policy; return XQC_OK = decision made (including explicit NONE no-op); return non-OK = fall back to auto